The increasing demand for electronic devices that are smaller, lighter, and yet more functional has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting footprints, yet which are capable of increased component packaging densities. One approach to satisfying this demand has been the development of techniques for stacking the semiconductor dies or chips contained in the package on top of one another. An example of die-stacking techniques is disclosed, for example, in U.S. Pat. No. 6,650,019 to T. P. Glenn et al.
FIG. 1 is a cross-sectional side elevation view of a prior art semiconductor package 100 incorporating two stacked dies 102 and 104. The package 100 illustrated is a ball grid array (“BGA”) package, having balls of solder 106 formed on the bottom surface of the substrate 110 that function as input/output terminals of the package. The package 100 includes a conventional interconnective substrate 110 and a first semiconductor die 102 mounted on a top surface of the substrate 110. A second die 104 has been stacked, i.e., mounted, on top of the first die 102. The dies 102 and 104 typically include a plurality of input/output wire bonding pads 112 located at the peripheral edges of their respective top surfaces.
The substrate 110 may comprise a flexible resin tape, a rigid fiber-glass/copper sheet laminate, a co-fired ceramic coupon, or a metal lead frame, all of known types in the industry, depending on the particular type of semiconductor package 100. The connective substrate 110 illustrated in the BGA package 100 shown in FIG. 1 comprises a layer 114 of an insulating material, e.g., a polyimide resin film, laminated between conductive layers 116 and 118, each of which comprises a metal, e.g., copper or aluminum, and makes up the respective top and bottom surfaces of the substrate.
The conductive layers 116 and 118 are typically patterned, e.g., by photolithography and etching techniques, to define wire bonding pads 120 and circuit traces in the top layer 116, and solder ball mounting lands 122 in the bottom layer 118. The wire bonding pads 120 and traces (not illustrated) are typically connected to the solder ball lands 122 through the thickness of the insulative layer 114 by vias 123, e.g., plated-through holes in the layers. Either or both of the conductive layers 116 and 118 may be coated with an insulating solder mask (not illustrated) that has inside openings, through which the respective metal pads for wire bonding 120 and/or solder ball lands 122 are exposed, and which serve to prevent bridging between the pads and/or lands by accidental solder splashes.
In FIG. 1, the first die 102 is conventionally mounted on the top surface of the substrate 110 with, e.g., a layer of an adhesive or an adhesive film 124. The first die 102 is electrically connected to the substrate 110 by a plurality of fine, conductive wires 126, typically gold or aluminum, which connect the pads 112 on the die 102 and the pads 120 on the substrate 110.
The second die 104 is mounted on the top surface of the first die 102 with an adhesive layer or film 128 that generally has a lateral perimeter positioned within the central area of the top surface of the first die, and inside of the peripheral wire bonding pads 112 thereon. That is, the adhesive layer 128 generally does not contact or cover either the wire bonding pads 112 or the conductive wires 126 bonded thereto. The adhesive layer 128 positions the second die 104 a sufficient distance above the first die 102 to prevent the second die from contacting the conductive wires 126 that are bonded to the first die 102. This helps prevent shorting out or breaking the bonding pads 112 and wires 126, and thus defines a peripheral space 130 between the two dies that extends around the entire perimeter of the adhesive layer 128. The second die 104 may be wire bonded to the substrate 114 in the same fashion as the first die 102. One or more additional dies (not illustrated) may be stacked in tandem on top of the second die 104 using the same technique.
FIG. 2 is a cross-sectional side elevation view of a prior art package having two stacked dies 150 and 152 with an alternatively filled adhesive 154. If, for example, the die-attach equipment lacks precise control, it may be desirable to include an alternate mechanism for precisely controlling the final bond line thickness of the adhesive layer 154 distributed between the opposing surfaces of the two dies 150 and 152. As shown in the FIG. 2, this may be achieved by filling the uncured, fluid adhesive 154 with a quantity of microspheres 156, each having a diameter approximately equal to the desired final thickness of the adhesive layer 154. As illustrated in FIG. 2, the second die 152 is pressed down onto the filled adhesive 154 until the bottom surface of the second die 152 bottoms out on the microspheres 156. The bottom surface of the second die 152 is spaced apart from the top surface of the first die 150 by approximately a single layer of the microspheres 156.
The material of the microspheres 156 may be selected from a wide array of materials such as glass, polymer, silicon dioxide, silicon nitride, or polytetraflouroethylene (“PTFE”). The microspheres 156 may be fabricated using a variety of known techniques such as pumping or blowing a molten material through a nozzle under high pressure to atomize it, then cooling or curing the varying-sized spherical bodies thereby produced with a bath of, e.g., air, water or oil. The microspheres 156 then may be passed through a series of screens of graduated mesh sizes to grade them by diameter.
With reference back to FIG. 1, in the stacked-die package 100 the dies 102 and 104 generally are wire bonded sequentially, typically with automated wire bonding equipment employing well-known thermal-compression or ultrasonic wire bonding techniques. As shown in FIG. 1, during the wire bonding process the head 132 of a wire bonding apparatus applies a downward pressure on a conductive wire 126 held in contact with a wire bonding pad 112 on the die to effect a weld or bond of the wire to the pad.
Because the wire bonding pads 112 are located in the peripheral area of the respective top surfaces of the two dies, the wire bonding generally entails the application of a relatively large, localized force in the direction of the arrow shown in FIG. 1 to the outside portion of the die. This generally does not present a problem with the bottom die 102 as it is supported from below by the substrate 110 and the adhesive layer 124. In the case of the second, top die 104, however, its peripheral portion is cantilevered out over the peripheral portion of the bottom die 102 by the adhesive layer 128, and is therefore unsupported from below. Consequently, the top die 104 may crack or fracture during the wire bonding procedure, which may result in the entire assembly being rendered unusable.
Another problem that may result from prior art die stacking techniques also relates to the peripheral space 130 created between the opposing surfaces of the first and second dies 102 and 104, as well as the perimeter of the adhesive layer 128. In particular, the plastic molding material used to form the body 134 that encapsulates the dies generally penetrates into the peripheral space during the molding process and forms a wedge between the two dies. If the encapsulating material has a thermal coefficient of expansion different from that of the adhesive spacer 128, it is possible for this wedge to expand within the peripheral space 130 under large changes in temperature of the package 100, thereby potentially fracturing one or both of the dies, again resulting in a defective package.
Another disadvantage of the prior art die stacking techniques also relates to the peripheral space 130 created between the opposing surfaces of the first and second dies 102 and 104 and the force exerted by the wire bonding head 132. In particular, the downward force of the head may deform the second die 104, thereby inducing separation or delamination of the bottom surface of the top die 104 and the top surface of the second adhesive layer 128.
Another disadvantage of the prior art die stacking techniques again relates to the peripheral space 130 and the deflection caused by the wire bonding head 132. In particular, deformation may occur within the conductive wires 126 or within the wire bond pads 112, sometimes causing immediate breakage. Alternatively, repeated deformation and flexure of the die during the bonding process generates vibrations through out the semiconductor package, in which the electrical connections may be fatigued due to the vibrations. While fatigue may not cause immediate breakage during manufacture, electrical component fatigue may reduce the lifetime and the reliability of the device.